Walter J. Lackey

Fabrication of carbon/carbon composites by forced flow-thermal gradient chemical vapor infiltration

The fabrication of carbon/carbon composite disks using forced flow-thermal gradient chemical vapor infiltration process using propylene diluted with hydrogen as the reagent is disclosed. The independent variables included concentration of the reagent, total flow rate and preform bottom temperature. The response variables included infiltration time, final porosity, rate of weight gain and deposition rate. The infiltration time and rate of weight gain are affected only by the three independent variables. The densification of the composites is monitored by the back pressure. The composites were cut into slices 1 cm long, 4 mm wide and 2 mm deep to measure the uniformity of densification, which varied very little within a composite. Coating thickness as a function of position increased exponentially from the cold side to the hot side of the composite. The coating thicknesses near the cold side and the hot side are used to calculate the rate constant for the pyrolysis of propylene in the preform. The activation energy is approximately 18.6 Kcal/mol and the rate constant is given by 1n k=8.2--9375.1/T (K)

1-D model for forced flow-thermal gradient chemical vapor infiltration process for carbon/carbon composites

A one-dimensional model was developed for the forced flow-thermal gradient chemical vapor infiltration of carbon/carbon composites. The infiltration time predicted by the model agreed very well with experiments, where propylene and propane were used as the hydrocarbon source. The model was also validated by interrupting the infiltration and comparing predicted with observed densities.

Chemical vapor deposition of silver films for superconducting wire applications

Abstract Chemical vapor deposition (CVD) was used to deposit silver films for superconducting wire applications. AgI, silver trifluoroacetate (Ag(TFA)), and perfluoro-1-methylpropenylsilver (Ag(PF)) produced the most promising silver films. CVD processing was optimized on these three precursors using thermodynamic calculations performed using a modified version of the SOLGASMIX-PV computer program. Ag(PF) produced the highest quality silver films at low temperatures and pressures. A fiber tow which contained a silver barrier layer and a YBa2Cu3Ox overlayer was found to be a superconductor at 72 K.

Model for prediction of matrix microstructure for carbon/carbon composites prepared by forced flow-thermal gradient CVI

A model has been developed to predict the microstructure of the carbon matrix deposited during forced flow-thermal gradient chemical vapor infiltration (FCVI). This method employs previous thermodynamic-microstructure and densification models to determine conditions throughout the preform as a function of time. The model was verified by comparison with samples prepared over a range of deposition temperatures, reagent concentrations and flow rates. The model also showed that it should be possible to deposit a matrix of uniformly high thermal conductivity onto conventional size carbon fibers, as well as small diameter, low cost, high thermal conductivity carbon whiskers.

Rapid Processing of Carbon-Carbon Composites by Forced Flow-Thermal Gradient Chemical Vapor Infiltration (FCVI)

Carbon fiber-carbon matrix composites were fabricated using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. Preforms were prepared by stacking 40 layers of plain weave carbon cloth in a graphite holder. The preforms were infiltrated using propylene, propane, and methane. The present work showed that the FCVI process is well suited for fabricating carbon-carbon composites; without optimization of the process, the authors have achieved uniform and thorough densification. Composites with porosities as low as 7% were fabricated in 8--12 h. The highest deposition rate obtained in the present study was {approximately}3 {micro}m/h which is more than an order of magnitude faster than the typical value of 0.1--0.25 {micro}m/h for the isothermal process. It was also found that the use of propylene and propane as reagents resulted in faster infiltration compared to methane.

Thermodynamic analysis for the chemical vapor deposition of composite coatings from the Al–B–Ti–N–H–Cl–Ar system

Thermodynamic calculations were performed for chemical vapor deposition in the Al–B–Ti–N–H–Cl–Ar system in order to determine the feasibility of multiphase deposition. Reagent species used were BCl 3 , AlCl 3 , TiCl 4 , NH 3 , H 2 , and Ar; B 2 was substituted for BCl 3 to determine changes in deposition efficiency. Temperature and input molar concentrations were varied over a range of values to establish relationships among solid deposits. Through deposition diagrams, molar efficiency plots, and partial pressure graphs, several two and three phase regions were found to exist. The calculations indicate that the following dispersed phase composites could be prepared: A1N + BN + TiN, BN + TiN, BN + TiB 2 , BN + TiB 2 + TiN, and TiB 2 + TiN.

The chemical vapor deposition of dispersed phase composites in the B-Si-C-H-Cl-Ar system

The CVD of the coatings in the B-Si-C-H-Cl-Ar system was accomplished using a statistically designed experiment. The experimental design used five factor half-fraction factorial with a central composite design that was both rotatable and orthogonal. Deposits were thick and dense and were composed of B{sub 13}C{sub 2} and {beta}-SiC with compositions ranging from 0 to 100%. Response surfaces were generated using multivariate regression for unit cell volumes of B{sub 13}C{sub 2} and {beta}-SiC, %B{sub 13}C{sub 2}/%SiC in the coating, and the Si to B ratio in the deposit. These equations could then be used to examine the significant variables in the reaction, as well as for tailoring and optimizing the deposition process.

Grain structure and growth of dispersed phase BN-AlN coatings grown via chemical vapor deposition

This paper discusses the variation in microstructures encountered during the separate depositions of boron nitride (BN) and aluminium nitride (AlN) as well as during the codeposition of BN-AlN dispersed phase ceramic coatings. This combination was chosen in order to take advantage of the self lubricating properties of hexagonal BN along with the hard, erosion resistance of AlN. Films were characterized using scanning and transmission electron microscopy (SEM and TEM), x-ray photoelectron spectroscopy (XPS), and x-ray diffraction (XRD). A range of coating microstructures are possible depending on the conditions of deposition. The best films produced, in terms of hardness, density, and tenacity, were a fine mixture of turbostratic BN and preferentially oriented A1N whiskers aligned with the whisker axis perpendicular to the substrate surface as seen by both electron microscopy and x-ray diffraction. 4 refs., 9 figs., 1 tab.